This invention relates to pneumatic devices which transmit a pressure signal in response to a sensed condition. More particularly, it relates to a force-balance pressure transmitter which produces a signal in response to the combined effect of sensed temperature and humidity.
Force-balanced pressure transmitters are well-known and operate by balancing a force proportional to the controlled pressure of the fluid against a control force generated by a condition being sensed. Such force-balance transmitters are thus distinguished from position-sensitive devices in which the measured condition is converted into a variable, relatively inflexible position of a means controlling fluid pressure. The force due to the controlled pressure constitutes a negative feedback that tends to linearize the relationship of the fluid pressure to the variable measured condition. In a typical pressure transmitter, a nozzle and flapper may be employed in a manner such that a control force exerted by the flapper in response to a sensed condition is exposed by a feedback force due to the fluid pressure acting over the area of the nozzle. Such a transmitter is described in my U.S. Pat. No. 3,452,928.
In certain applications it is desirable to determine the total heat or enthalpy of a fluid. For example, in a comfort conditioning system it is typical to utilize outside air to a certain extent in providing properly conditioned air for distribution to various zones. For purposes of energy conservation it is important to determine how much outside air should be used to minimize energy consumption. The energy required to condition air is proportional to the enthalpy of the air. Thus, by comparing the enthalpy of the outside air to that of the return air in the conditioning system a determination can be made as to the amounts of outside air and return air which should be used to accomplish the desired conservation of energy. The enthalpy of air is the sum of the enthalpy of the dry air plus the enthalpy of the moisture present in the air. The enthalpy of dry air is proportional to the temperature of the air and this enthalpy can be determined by utilizing a temperature sensor. The enthalpy of the moisture in the air is proportional to humidity. Thus the moisture enthalpy can be determined by utilization of a humidity sensor. Consequently, it is necessary to combine the outputs of the temperature and humidity sensors to obtain a control signal proportional to the enthalpy of the moist air.
Another application in which it is desirable to employ a transmitter which combines sensed temperature and sensed humidity is in the determination of the dew point of air. In a given zone it may be critical to prevent condensation which could damage the contents in the zone and thus it would be important to maintain the temperature above the dew point. The dew point depends on the temperature and relative humidity of the air. Properly combining the outputs of temperature and humidity sensors will result in a signal indicative of the dew point.